Synthesis, characterization and application of oxovanadium(iv) complexes with [NNO] donor ligands: X-ray structures of their corresponding dioxovanadium(v) complexes

Two oxovanadium(iv) complexes ligated by [NNO] donor ligands have been synthesized and characterized by ESI-HRMS, elemental (CHN) analysis and spectroscopic (UV-Vis, IR and EPR) techniques. Block shaped brown crystals from the methanolic solutions of these oxovanadium(iv) complexes were obtained during the crystallization process. Crystallographic structures of the resulting crystals revealed that the original oxovanadium(iv) complexes have been transformed into new dioxovanadium(v) complexes with concomitant oxidation of VIV to VV. The original oxovanadium(iv) complexes have been identified to be an efficient catalyst for the CO2 cycloaddition reaction with epoxides resulting up to 100% cyclic carbonate products. The geometries of oxovanadium(iv) complexes are optimized by the density functional theory (DFT) calculations at the uB3LYP/6-31G**/LANL2DZ level of theory. The geometry and structural parameters of optimized structures of oxovanadium(iv) complexes are in excellent agreement with the parameters of X-ray structures of their dioxovanadium(v) counterparts. Further, TD-DFT and Spin Density Plots for the oxovanadium(iv) complexes are performed in order to get more insights about their electronic absorption and EPR spectroscopies, respectively.


Introduction
Carbon dioxide, a major greenhouse gas, is being released into the Earth's atmosphere in a large scale due to the combustion of fossil fuels leading to global warming, which is at present posing a serious environmental threat. [1][2][3][4][5] Nevertheless, fossil fuel burning is still inevitable due to the increasing demand of energy and lack of feasible technologies for sustainable energies. 1 Therefore, the strategies that need to be immediately adopted are the reduction in carbon dioxide emission and the attenuation of carbon dioxide concentration in the atmosphere. On the other hand, CO 2 is an abundant, renewable, inexpensive and non-toxic C1 source of chemical carbon in organic syntheses. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Although utilization of carbon dioxide as C1 feedstock is challenging in view of its thermodynamic and kinetic inertness, 16 research that deals with sustainable use of CO 2 in producing value-added chemicals is advancing at a rapid rate. [17][18][19][20][21] Of many important CO 2 transformation reactions, 22 cycloaddition of CO 2 with epoxides to form cyclic carbonates stands out to be a very attractive transformation process.  The process is 100% atom economic and the resulting cyclic carbonate products are commercially important chemicals which can be used as electrolytes in lithium-ion battery, 58 polar aprotic solvents, 59 functionalized building blocks for synthesizing valuable organic products, 60 monomers for polycarbonates 61 and isocyanate-free polyurethanes, 62 intermediates for pharmaceuticals and ne chemicals. 63 Interestingly, cyclic carbonate moieties are also seen in some natural products [64][65][66] thereby making the CO 2 transformation reaction to cyclic carbonate a very signicant and naturally relevant reaction.
Herein, we report synthesis, characterization and the catalytic study of substituted salicylidin-2-picolylimine ligated vanadium complexes in the CO 2 cycloaddition with epoxides yielding cyclic carbonates with very good to excellent conversions under relatively benign conditions. Complex 1 was prepared by rst reuxing an equimolar mixture of 5-bromosalicylaldehyde and 2-picolylamine in ethanol for 2 h, followed by addition of aqueous VOSO 4 solution and subsequent reuxing for another 1 h. Green solid complex 1 was obtained when an aqueous solution of sodium nitrate was added to the former solution and stirred for 1 h at room temperature. Dioxovanadium(V) complex, 1A was obtained as brown blocked shaped crystals from the methanolic solution of complex 1 aer around two weeks. Similarly, complex 2 has been synthesized in one pot by reuxing equimolar mixture of 4-(diethylamino)salicylaldehyde and 2-picolylamine in methanol for 2 h and subsequently reuxing further for 3 h aer addition of 1 equivalent [VO(acac) 2 ]. The solution obtained was ltered and from the ltrate greenish color solid is collected and dried. Dioxovanadium(V) complex, 2A was obtained as brown blocked shaped crystals from the methanolic solution of complex 2 aer around one week.

X-ray crystallography
Single crystals suitable for X-ray crystallographic analysis of However, crystals of complexes 1 and 2 could not be obtained in spite of repeated attempts. Brown block shaped crystals of 1A suitable for X-ray diffraction were obtained via slow evaporation of solution containing complex 1 in methanol for around 2 weeks. Complex 1A crystallises in a monoclinic crystal system with P2 1 /c space group. Perspective view of the asymmetric unit of complex 1A and the crystal packing are illustrated in Fig. 1 and Fig. S1 (ESI †), respectively. Crystal data collection parameters and the selected bond distances and angles are shown in Table S1 (12) .
Brown block shaped crystals of 2A were grown via slow evaporation of solution containing complex 2 in methanol for around 7 days. Complex 2A crystallises in a monoclinic crystal system with P2 1 space group. Perspective view of the asymmetric unit of complex 2A and the crystal packing are illustrated in Fig. 2 and S2 (ESI †), respectively. Asymmetric unit of the crystal contains two symmetry-independent molecules viz A and B. It is observed that molecules are crystallographically non-equivalent as the corresponding bond distances and angles differ slightly in these two molecules (Table S3, (Table 1) are similar with the experimentally determined X-ray structures of their dioxovanadium(V) counterparts, 1A and 2A.

NMR spectroscopy
In spite of repeated attempts, no good 1 H NMR spectrum of complex 1 was obtained, may be due to the presence of paramagnetic vanadium(IV) center. 69 However, 1 H NMR spectrum of complex 2 is reasonably good (Fig. S3 †) although the signals are broad. The resonance at 8.80 ppm is due to the imine (-CH]N) proton. There appear four signals (8.51, 7.98, 7.61 and 7.44 ppm) for picolyl aromatic protons. The resonance at 5.33 ppm can be attributed to the picolyl methylene proton. Moreover, the phenolate proton signals appear at 7.22, 6.20 and 5.90 ppm. It is worth mentioning that -CH 2 protons of ethyl groups and -OCH 3 protons resonate in the same region with residual water impurity.

EPR spectroscopy
EPR spectrum (     Bond lengths (Å) in Fig. 6 reveals the localisation of single spin (S ¼ 1/2) on vanadium(IV) center with no spin delocalisation over ligand, thus supporting the experimental EPR signal. 71,72 Expectedly, similar kind of EPR spectrum (Fig. 7) is also observed for the oxovanadium(IV) complex 2 (g iso ¼ 1.9935, A iso ¼ 96.557 G). Moreover, the spin density plot of the complex 2 ( Fig. 8) also indicates the localisation of spin density on oxovanadium(IV) center like that in complex [1 -NO 3 À ] + .

IR spectroscopy
The IR spectra of oxovanadium(IV) complex 1, Fig. S4 and S5 (ESI †), respectively. The IR spectrum of oxovanadium complex 1 displayed a strong band at 964 cm À1 that can be attributed to n(V]O) of the vanadyl moiety present in the complex. 73 Moreover, the peak at 1627 cm À1 is related to vibration of the azomethine moiety (C]N). 74 Further the peaks at 1761, 1383 and 830 cm À1 are attributable to the NO 3 À . 75 In a similar way, the IR spectrum of oxovanadium complex 2 reveals the formation of the complex which shows a peak at 924 cm À1 corresponding to n(V]O) stretching of the vanadyl group. Further the strong band observed at 1596 cm À1 corresponds to n(C]N) of the azomethine group. 76

UV-visible spectroscopy
The UV-visible spectrum of oxovanadium(IV) complex 1 (5.0 Â 10 À5 M) in dichloromethane have been illustrated in Fig. 9. UVvisible spectrum of 1 displays a peak in the higher energy region at 331 nm, which is due to ligand centred transitions. 77 Band appearing at 402 nm is attributed to the LMCT transitions originating from the p orbital of phenolate oxygen to the empty d orbital of vanadium(IV) center. 78 Moreover, d-d transition in complex 1 is apparent (500 and 660 nm) when the concentration of 1 was increased to 3.0 Â 10 À3 M. 79 Similarly, UV-visible spectrum of 2 (Fig. S6, † le) exhibits a band at 349 nm corresponding to p-p* transition of azomethine chromophore. 80,81 The intense band observed at 402 nm can be attributed to the charge transfer transition (LMCT) from the p orbital of the phenolate oxygen to empty d orbital of vanadium(IV). 78 However, the bands due to d-d transitions could not be distinguished properly as they are probably under the tail of much stronger LMCT band at ca. 465 nm (shoulder). 82 Moreover, the UV-visible spectrum (Fig. S6, † right) recorded in DMF solvent of dioxovanadium(V) complex 2A shows similar absorption pattern showing LMCT bands at 396 and 465 nm (shoulder) as that of oxovanadium(IV) complex 2, suggesting that complex 2 get oxidised to complex 2A.     The time-dependent density functional theory (TD-DFT) calculation of [1][2][3] ] + shown in Fig. 10 indicates the presence of absorption band at longer wavelength (546 nm; oscillator strength f ¼ 0.1281) that can be attributable to the dd transition.

Cycloaddition reaction of CO 2 with epoxides
CO 2 cycloaddition with epoxides for the formation of cyclic carbonates was performed by loading vanadium(IV) complexes 1 or 2 with tetrabutylammonium bromide (TBAB) co-catalyst in a stainless-steel autoclave equipped with a magnetic stirring bar and dosing appropriate pressure of CO 2 (Scheme 2, Table 2). The formation of cyclic carbonates has been conrmed by 1 H NMR ( Fig. S10-S22, ESI †) showing downeld shiing of the peaks compared to epoxides because of the addition of CO 2 . The formation of cyclic carbonate products was also supported by the IR spectroscopic technique, which showed characteristic peaks in a range of 1783-1786 cm À1 that correspond to n(C]O) of carbonate moiety (Fig. S23-S25 †). Preliminary investigation of the potentiality of the catalyst system 1-TBAB (1 mol%: 2 mol%) was performed at 60 C and 5 bar CO 2 pressure taking epichlorohydrin (ECH) and the epibromohydrin (EBH) as the substrates. To our delight, quantitative conversions of ECH (entry 1, 100% conv.) and EBH (entry 2, 100% conv.) to their corresponding cyclic organic carbonates (COCs) were achieved. To see the catalytic activity of the oxovanadium(IV) complex 1 alone, we have performed the reaction in the absence of TBAB co-catalyst (entry 3). However, it is revealed that vanadium complex 1 alone does not have any capability to transform the styrene oxide (SO) into its corresponding COC. To see the catalytic efficiency of the complex 2, a similar kind of reaction was performed taking ECH as substrate and found 2-TBAB catalyst system to have the same extent of efficiency (entry 4, 100% conv.) as that of 1-TBAB. To see the substrate scope of the catalyst systems, we have chosen three other substrates viz. styrene oxide (SO), allyl glycidyl ether (AGE) and butyl glycidyl ether (BGE). Both the catalyst systems 1-TBAB and 2-TBAB showed similar kinds of conversions when SO was taken as substrate (compare entry 5 and 6, 95% and 93% conv., respectively). Comparing the entry 5 (95% conv.) and entry 7 Scheme 2 Catalytic cycloaddition reaction of CO 2 with epoxides using oxovanadium(IV) complexes 1 and 2 in presence of co-catalyst TBAB. (26% conv.), it is observed that 1-TBAB catalyst system is superior to TBAB alone, as more than around 68% conversion was found when the combination of complex 1 and TBAB was used as a catalyst system instead of TBAB alone. Catalyst system 2-TBAB appears to be a bit better than 1-TBAB in case of the substrate AGE (compare entry 8 and 9, 85% and 92% conv., respectively). For the substrate BGE, both the vanadium complexes 1 and 2 showed similar potentiality as catalysts resulting in 85% (entry 10) and 82% (entry 11) conversions for catalyst 1 and 2, respectively. To investigate the effect of CO 2 pressure on the catalytic activity of the catalyst system, a reaction was performed with SO as substrate under 1 bar of CO 2 pressure keeping other parameters the same. It is seen that 100% conversion of SO (entry 12) to its corresponding COC could be achieved within 6 h of duration in a mere 1 bar of CO 2 pressure indicating 1-TBAB combination to be a very efficient catalyst.

Catalytic cycle
As suggested from previously reported work 52 on metal complex catalysed conversion of carbon dioxide and epoxides into cyclic carbonates, an attempt has been made to sketch a plausible reaction pathway for cycloaddition of carbon dioxide to epox-

Materials
All chemicals used in this work were purchased from Sigma Aldrich, Spectrochem, and Alfa Aesar, and were used without further purication.

Instrumentation
IR spectra were recorded on a BRUKER infrared spectrometer (model no ALPHA II). Elemental analysis (CHN) measurements were done in Thermo Scientic FlashSmart CHNS/O analyzer. The electronic spectra were recorded using a Shimadzu UV-2600 spectrophotometer. 1  constant, 0.03 s]. The crystals of complex 1 and 2 were coated with light hydrocarbon oil and mounted at 273(2) K and 296 (2) K, respectively on a Bruker SMART APEX CCD diffractometer, and the intensity data were collected using graphitemonochromated Mo Ka radiation (l ¼ 0.71073Å). The data integration and reduction were processed with SAINT soware. 83 An absorption correction was applied. 84 The structure was solved by the direct method using SHELXT 2014/5. The structure was rened on F 2 by the full-matrix least-squares technique using the SHELXL-2018/3 program package. 85 Nonhydrogen atoms were rened anisotropically. In the renement, hydrogens were treated as riding atoms using SHELXL default parameter. 4.2.1 Computational details. Gaussian 09, revision B.01, program 86 was used to carry out the DFT calculations and the method used was Becke's three-parameter hybrid-exchange functional, [87][88][89] with the nonlocal correlation provided by the Lee, Yang, and Parr expression and the Vosko, Wilk, and Nuair 1980 correlation functional (III) for local correction. 90,91 Geometry optimizations were executed in which all of the coordinates were taken from the molecular structures wherever possible. Geometry optimizations were carried out for both the complexes [1 À NO 3 À ] + and 2 using the basis set 6-31G** for the C, N, O and H atoms and LANL2DZ for the V atom while excluding the counter anion for the calculations. To conrm that the optimized geometries did not have imaginary frequencies, frequency calculations were performed. In order to consider the solvent effect (dichloromethane was used as the solvent), self-consistent reaction eld method was applied in all of the calculations. Zero-point energies and thermal corrections were also included. As no imaginary frequencies were found, the optimized geometry was ensured to be the potential energy minima by vibrational frequency calculations at the same level of theory. The Chemcra soware program 92 was used to visualize the orbital surfaces. All of the TD-DFT calculations were performed at the CAM-B3LYP level. To this solution was added 0.163 g (1 mmol) of vanadyl sulphate monohydrate by dissolving in 20 mL of distilled water and stirred for another 1 h at room temperature. Then a solution of 0.085 g (1 mmol) sodium nitrate in 5 mL water was added and again stirred for another 1 h to get a green precipitate. The reaction mixture was then ltered and the precipitate obtained was washed with little amount of water and dried in a desiccator to produce green color solid of complex 1. Yield (0.281 g, 75%). Anal. calcd (found) for C 13  . A solution of 0.216 g (2 mmol) 2-picolylamine dissolved in 20 mL methanol was taken in a round bottom ask and added to it a solution of 0.386 g (2 mmol) 4-(diethylamino)salicylaldehyde in methanol dropwise. The contents of the ask were then reuxed with stirring for about 2 h and then a solution of VO(acac) 2 (0.532 g, 2 mmol) in 25 mL methanol was added and the reaction mixture was further reuxed for another 3 h with continued stirring. Aer cooling to room temperature, the reaction mixture was ltered and evaporation of the ltrate in open air afforded greenish color solid of complex 2. Yield (0.315 g, 83%). Anal. calcd (found) for C 18

General procedure for catalytic reactions yielding cyclic carbonates
The oxovanadium(IV) complex 1 or 2 (1 mol%), TBAB (2 mol%) and epoxide (0.5 mL) were mixed together in a 100 mL stainless steel autoclave and the existing air in the reaction vessel was evacuated with a ush of CO 2 gas for some time. The reaction vessel was then pressurized with appropriate pressure of CO 2 and stirred for desired time at 60 C. Upon completion of the catalytic reaction, the residual CO 2 of the autoclave was vented out carefully. Aer cooling the autoclave to room temperature, the reaction mixture was collected for analysis.